(CNN) --Underscoring its original ruling last November, Massachusetts' highest court said Wednesday that only full marriage rights for gay couples, not civil unions, would conform to the state's constitution.

The ruling sets the stage for Massachusetts later this year to likely become the first state in the nation to allow same-sex marriages.

In a statement released Wednesday night, President Bush said the ruling was "deeply troubling.

"Marriage is a sacred institution between a man and a woman," Bush said. "If activist judges insist on re-defining marriage by court order, the only alternative will be the constitutional process. We must do what is legally necessary to defend the sanctity of marriage."

In his State of the Union address January 20, the president stopped short of endorsing a constitutional amendment that would ban marriages for gay and lesbian couples, as social conservative groups had hoped.

But he said, "... if judges insist on forcing their arbitrary will upon the people, the only alternative left to the people would be the constitutional process."

Wednesday's advisory opinion by the Massachusetts court was in response to a request from that state's Senate about whether allowing gays to join in civil unions would be sufficient.

The court rejected using civil unions as a remedy, "Because the proposed law by its express terms forbids same-sex couples entry into civil marriage, it continues to relegate same-sex couples to a different status. ... The history of our nation has demonstrated that separate is seldom, if ever, equal."

Governor calls for vote

Wednesday's ruling came a week before a constitutional convention will be held by state lawmakers to consider an amendment legally defining marriage as a union between men and women. That amendment would have to be ratified by both houses of the Legislature in two successive legislative sessions and then be ratified by voters.

The earliest voters could consider a constitutional amendment would be November of 2006. The Massachusetts high court ruling from last November and reiterated on Wednesday will become state law in mid-May, regardless of what the constitutional convention decides.

"The people of Massachusetts should not be excluded from a decision as fundamental to our society as the definition of marriage," said Massachusetts Gov. Mitt Romney in a written statement on Wednesday's opinion. "This issue is too important to leave to a one-vote majority of the [Supreme Judicial Court]."

In November, the Massachusetts high court cleared the way for lesbian and gay couples in the state to marry, ruling 4-3 that commonwealth attorneys "failed to identify any constitutionally adequate reason" to deny them the right. The November 18 ruling gave the Legislature six months to rewrite the state law to conform to the ruling. (Full story)

The state Senate then asked the court whether the commonwealth could satisfy its constitutional concerns by granting civil unions to gays and lesbians, but forbidding them from obtaining civil marriage licenses.

Civil unions grant couples most of the rights of state civil marriages, except the name, but provide none of the federal benefits of marriage, such as Social Security benefits.

Decisions ignite political debate

Romney: Issue "too important to leave to a one-vote majority of the [Supreme Judicial Court]."

The November ruling in Massachusetts, as well as the granting of marriages to gay couples in Canada, set off a debate in the United States and among the Democratic presidential candidates.

Front-runner Sen. John Kerry is from Massachusetts and does not support gay marriage but does support civil unions. Gov. Howard Dean is from Vermont, which created civil unions.

Senate Democratic Leader Tom Daschle of South Dakota has said he believes the Defense of Marriage Act provides enough protection for the definition of marriage and that an amendment is not necessary.

Last year, California's State Assembly passed a domestic partnership law to provide similar benefits, but that law stops short of allowing gays to marry. Several other states have granted limited marriage benefits to gays but called them domestic partnerships.

White House lawyers have been studying the legal implications of the Massachusetts decision in light of the federal Defense of Marriage Act and a possible constitutional amendment.